1. Field of the Invention
The present invention relates to an image forming apparatus which directly regulates and modulates a flow of toner particles and forms toner images on image recording medium.
2. Description of the Related Art
Currently, an image forming apparatus of a type that jets toner particles onto image recording medium to record thereon toner images has been proposed, for example, in U.S. Pat. No. 3,689,935. According to the disclosure of this patent, the image recording apparatus includes a toner supply portion, an aperture electrode having a plurality of apertures, and a back electrode. The toner supply portion produces electrically charged toner particles, and provides the electrically charged toner particles toward the aperture electrode.
The aperture electrode for regulating and modulating the flow of toner particles includes an insulating layer, a first electrode layer coated on one side of the insulating layer, and a second electrode layer coated on the side of the insulating layer oppositing to the first electrode layer. The aperture electrode is formed with apertures or through-holes penetrating through the first electrode layer, the insulating layer and the second electrode layer. The second electrode layer includes multiple isolated segment electrodes which respectively surround each of the apertures.
The aperture electrode thus constructed controls the flow of the toner particles by selectively applying electric signals to, thereby producing an electric charge at, the isolated segments of the second electrode layer. Toner particles are selectively attracted to the isolated segments that are thus electrically charged and pass through the apertures surrounded by the isolated segments.
The back electrode is positioned to confront the aperture electrode for electrostatically attracting electrically charged toner particles which have passed through the aperture electrode. The back electrodes attracts and supports an image recording medium such as paper, to which the toner particles impinge and cling, forming toner images.
During the above-described recording process, however, the toner particles are liable to adhere to and block the apertures of the aperture electrode. More particularly, to obtain a dot diameter of less than 100 .mu.m, which is the maximum dot diameter necessary for obtaining an image density of 240 DPI (dot per inch), the inner diameter of the apertures must be approximately 50 .mu.m or less. However, image forces (the electrostatic force on a charge in the neighborhood of a conductor, which may be thought of as the attraction to the charge's electric image) formed on the aperture electrode causes the electrically charged toner particles to adhere to all the areas of the aperture electrode including in and around the apertures. The toner particles thus adhering in and around the apertures accumulate in and block the apertures, making output images irregular.
To solve this problem, one of the co-inventors of the present invention has proposed in co-pending U.S. patent application Ser. No. 07/783,248 filed Oct. 28, 1991, now U. S. Pat. No. 5,293,181, mechanism for preventing such blockage. The mechanism excites the aperture electrode with progressive waves having vibration acceleration sufficiently large to overcome the image forces that cause toner particles to adhere to the aperture electrode. In other words, the aperture electrode is applied with vibration acceleration which attains a repulsion force greater than the image force occurred in the aperture electrode.
The mechanism includes an oscillator installed on the aperture electrode for propagating the progressive waves. Because the aperture electrode is usually rectangular shaped with a plurality of apertures formed in a row running in the longitudinal direction of the aperture electrode, the oscillator is installed near one end of the row of apertures.
However, addition of this mechanism has created an additional problem in that reflected waves are generated in addition to the progressive waves. When the reflected waves and the progressive waves overlap, the waves interfere with each other, and therefore fluctuations in the amplitude of waves, including standing waves, appear. The vibration acceleration that toner particles are subjected to at nodes of the standing waves is insufficient for preventing their accumulation in the apertures.
To overcome this problem, the mechanism proposed in the co-pending U.S. patent application Ser. No. 07/783,248, filed Oct. 28, 1991, now U.S. Pat. No. 5,293,181, further provides an absorber, for absorbing the energy of the progressive waves, mounted at the end of the row of apertures opposing the oscillator and thereby preventing generation of reflected waves. More specifically, the absorber is provided to allow propagation of only progressive waves along the aperture electrode in the longitudinal direction. This insures that waves having a generally stable amplitude distribution in the longitudinal direction form on the aperture electrode. However, this absorber does not completely consume the energy of the progressive waves as required to prevent production of reflective waves. Completely consuming the energy of the progressive waves would require addition of complex devices, such as a device with its impedance being adjustable to completely match the impedance of the aperture electrode.